1. Field of the Invention
The present invention relates to a process for chemical desulfurization of coal.
2. Description of the Prior Art
Coal contains inorganic sulfur mainly composed of pyrite (FeS.sub.2) and organic sulfur mainly composed of aliphatic sulfur compounds such as mercaptan, sulfide and disulfide, and aromatic sulfur compounds such as thiophene. benzothiophene and dibenzothiophene. Therefore, when coal is combusted, sulfurous acid gas (SO.sub.2) is exhausted into the atmosphere to cause air pollution.
For preventing the exhaustion of the sulfurous gas into atmosphere, a flue gas desulfurization method is widely used. Flue caused by coal combustion is removed by wet-treating with an aqueous solution of an alkali such as calcium hydroxide, etc., to react the sulfurous gas with the alkali. Further, a process for chemical desulfurization of coal has been studied and developed in which coal is chemically treated to remove sulfur compounds from the coal before combustion. For example, JP-A-3-275795 and JP-A-3-275797 disclose a process for chemically removing sulfur compounds from coal, in which crushed coal is mixed with an aqueous solution of sodium hydroxide or potassium hydroxide and the mixture is heated in an oxygen gas atmosphere. This process includes a leaching step in which an oxygen gas is introduced as an oxidizing agent to convert the sulfur compounds into highly chemically reactive tetravalent sulfur oxides and these oxides are absorbed in alkali hydroxide to remove the sulfur compounds from coal. For example, crushed coal is subjected to oxidative alkali leaching treatment using a 0.5 to 10N alkali hydroxide aqueous solution, an oxygen gas, as an oxidizing agent, having a pressure of 1 to 30 kg/cm.sup.2 and a temperature of 250.degree. to 450.degree. C., whereby the coal is desulfurized to give coal having a low sulfur content.
However, the process for chemical desulfurization of coal, disclosed in the above publications has the following defects.
(i) Although the inorganic sulfur in the whole sulfur compounds in coal can be nearly completely removed, the organic sulfur is not fully removed. That is because the organic sulfur bonds to a coal matrix more strongly than the inorganic sulfur and is less reactive with an alkali as a desulfurizing agent. In particular, aromatic sulfur compounds such as thiophene and thiophene derivatives are stable themselves and strongly bond to a coal matrix, and these compounds therefore cannot be fully removed.
(ii) Since the oxidation reaction is carried out in an oxygen-containing atmosphere at a high temperature, the oxidation never occurs without oxidizing a coal matrix to decompose it. Thus, the coal properties are affected, and the yield (or recovery) of a low sulfur coal is low.